1. Field of the Invention
The present invention relates to a bump structure provided on a substrate of such element as a semiconductor element, and more particularly to a bump structure, a method and an apparatus for forming such a bump structure, and a bump structure used for flip-chip mounting.
2. Description of the Related Art
A bump structure formed using a solder 5 in a conventional manner is shown in a diagrammatic cross sectional view in FIG. 1C. On an A1 electrode 2 of a semiconductor element 1, a Cr layer is formed as a bonding layer 72 and a Cu, Ni layer is formed as a diffusion prevention layer 73. On the diffusion prevention layer 73, there is a bump of solder 5 in a spherical form. The solder 5 has a composition of Pb 95%-Sn 5% or a eutectic composition. A passivation film 3 is provided on the semiconductor element 1, and has an opening at the portion where the A1 electrode 2 of the passivation film 3 is disposed. FIGS. 1A and 1B are sectional views showing sequential steps of forming the bump structure shown in FIG. 1C. A metal mask 71 is positioned over the A1 electrode 2 of the semiconductor element 1 in its wafer state, and the sequential sputtering is performed for the Cr layer as the bonding layer 72 and the Cu, Ni layer as the diffusion prevention layer 73, followed by the vapor deposition of the Sn and Pb with the composition necessary for the solder 5--(FIG. 1A). Then, the metal mask 71 is removed (FIG. 1B), the solder 5 is heated to its melting point and, after the solder 5 is shaped spherically due to its surface tension, the resulting structure is diced and divided into IC pellets mountable as flip chips.
FIG. 2 shows another conventional bump structure in a state in which it is connected to an electrode 52 of the wiring substrate 51 by a conductive resin 62. This bump structure is formed by a wire bonding technique and is called "stud bump" 63 (disclosed in National Technical Report Vol. 39, No. 2, April 1993 published by Matsushita Electric Industrial Co., Ltd.) which is a double stepped structure and in which an Au ball is formed at a leading end of a bonding wire in a loop form. FIGS. 3A-3D are sectional views for explaining sequential steps of forming the stud bump 63. As shown in FIG. 3A, the Au ball is first formed at the leading end of the bonding wire 21. Then, through a capillary 22 and by an ultrasonic incorporated thermo-compression means, the Au ball is compressed to the A1 electrode 2 of the semiconductor element 1 as seen in FIG. 3B, whereby the bottom part of the stud bump is formed. Thereafter, the capillary 22 is moved along a locus 25 forming a loop shown in FIG. 3C and, as the capillary 22 is moved downward as shown in FIG. 3D, the bonding wire 21 is second-bonded to the bottom part of the stud bump and is cut, whereby the leading end is formed on the upper surface of the bottom part of the stud bump.
Next, for the flatness and the height uniformity of the upper surface, the resulting stud bump 63 is subjected to leveling by applying pressure with a flat surface on the upper surface of the stud bump 63. By applying a pressure of about 50 g/bump, it is possible to make the heights of the bumps uniform with high precision.
FIGS. 4A-4C show the steps of a conventional method in which the bumps are formed. FIG. 4A shows that a film of conductive resin 62 is formed in advance to the thickness about half the height of the bumps 63 on a transfer base 65. The tip portions of the stud bumps 63 are pressed into the film of conductive resin 62 (FIG. 4B) and, when raised, the conductive resin 62 touched are transferred simultaneously onto all of the tip portions of the stud bumps 63 (FIG. 4C). Then, as shown in FIG. 5, the semiconductor element 1 is turned upside-down and is mounted on the wiring substrate 51. Upon completion of the mounting, the conductive resin 62 is cured, and the gap between the semiconductor element 1 and the wiring substrate 51 is shielded with resin 53.
FIGS. 6A-6C show the steps of a conventional method in which a conductive resin material is provided on the bump 6a. FIG. 6D shows a semiconductor chip which has been mounted on the wiring substrate by utilizing the bumps.
FIG. 6A shows that the bump 6a is formed on the A1 electrode 2 of the semiconductor chip 1. As shown in FIG. 6B, a conductive resin film 7 having a thickness of 20.about.30 .mu.m is formed on a flat glass base 21, and the bump 6a of the semiconductor chip 1 is pressed into the conductive resin film 7 (FIG. 6B). When the semiconductor chip 1 is raised, the conductive resin film 7 touched is transferred onto the tip portion of the bump 6a (FIG. 6C). The conductive resin material 22 thus adhered to the tip portion has a diameter the same as or larger than that of the bump 6a. When the semiconductor chip 1 is pressed against the conductive resin film 7 on the glass base 21, the conductive resin film 7 surrounding the periphery of the bump 6a is caused to form a raised portion 23 and, depending on the magnitude of this raised portion, there arise variations in the quantity of the conductive resin material 22 that adheres to the bump 6a.
As shown in FIG. 6D, when the semiconductor chip 1 is mounted on the wiring substrate 5, the resin material 22 that adheres on the bump 6a spreads out on the wiring substrate 5, whereby the semiconductor chip 1 and the wiring substrate 5 are mechanically and electrically connected.
FIG. 7 shows another conventional method of attaching a conductive resin material on a bump, which is disclosed in Japanese Patent Application Kokai Publication No. Hei 1-143291. According to this method, a conductive resin material 7 is filled-in in advance in an opening 9 of a metal mask 10 and, after this opening 9 is brought into registry with the bump 6a of the semiconductor chip 1, the compressed air is ejected from a guided air opening 24, thereby causing the conductive resin material 7 to adhere to the bump 6a.
The problems existed in the structures resulted from the conventional methods explained above include the following.
For the conventional structure explained with reference to FIGS. 1A-1C, since such layers as the bonding layer 72 and the solder diffusion prevention layer 73 are formed on the A1 electrode 2 of the semiconductor element 1 by such methods as plating and vacuum deposition, the number of steps and the cost for such process are unavoidably increased. The plating method and the vacuum deposition method require a great investment in plant and equipment
Also, for the conventional structure explained with reference to FIGS. 3A-3D, since the formation of the wire loop is involved, the material that can be applied is limited to Au which is comparatively soft. Thus, between Au and Sn, a brittle compound metal is formed, and this cannot be used for the solder connection for the reasons of reliability. In the bump formation method, when the loop is formed, the wire tends to be loosened during the capillary driving because the diameter of the Au wire is smaller than the inner diameter of the tip of the capillary, and it is not possible to obtain uniform shapes of the loop. Also, depending on a lack of locational precision of the bonding device, there will be variations in the orientation of loops. Furthermore, the leveling which is made to make the heights of the bumps uniform will cause the bumps to be greatly deformed in a lateral direction. Thus, the amount of the conductive resin supplied will be varied so that, after the flip chip mounting, there may occur wire breakage or short-circuiting due to shortage of the solder or conductive resin. Also, the fact that the leveling is an additional step means that there is an increase in the cost of fabrication.
In the bump structure of the conventional semiconductor chip explained with reference to FIGS. 6A-6D, the diameter of the conductive resin material 22 that adheres to the bump 6a on the A1 electrode 2 of the semiconductor chip 1 is either substantially the same as or larger than that of the bump 6a. This is because the bump 6a is submerged directly into the conductive resin film 7. When the resulting semiconductor chip 1 is connected to the substrate electrode 26, the conductive resin material 22 spreads out further so that, where the substrate electrodes 26 are provided with narrow pitches, the electrodes adjacent to one another may easily suffer from short-circuiting problems due to the spread-out conductive resin 22. Also, where the semiconductor chip 1 is to be exchanged due to defects therein, the problem attendant is that, since the area in which the conductive resin material 22 is attached is large, a damage may be caused to occur when the semiconductor chip 1 is separated from the wiring substrate 5 both in the substrate electrode 26 and the semiconductor chip 1.
In the conventional method explained with reference to FIG. 7, the problem attendant is that, since the conductive resin material that adheres to the bump 6a spreads into and around the side of the bump 6a, it is difficult to attach the conductive resin material on the bump 6a in a uniform and stable manner.